Typically, a distributed amplifier is a simple but uniquely architected structure that fundamentally enables a broadened frequency of operation when compared to conventional architectures. Historically, distributed amplifiers tend to suffer in having limited output power capability or relatively low efficiency. Physical implementation of distributed amplifiers may be 1) on a printed circuit board (PCB) with discrete transistors, 2) completely internal to an integrated circuit (IC) package, or 3) a combination of these. The distributed amplifier can be viewed as having an input transmission line acting as a radio frequency (RF) input signal path, and an output transmission line for collecting and summing amplified RF signal components. The distributed amplifier embeds amplification sections connected between the input and output transmission lines for amplifying RF energy associated with the RF signal. The amplification section consists of several transistors or power-transistors periodically suspended between the input and output transmission lines.
In general, an input RF signal is supplied to a section of transmission line connected to an input of a first power transistor. As the input RF signal propagates along the input transmission line, the individual power transistors amplify the RF signal samples, and re-apply their amplified signal component to be collected and summed in the proper additive phase on the output transmission line. Thus, the energy continues to build on the output transmission line segments. If the application is a so-called high power distributed amplifier (DA), the currents in these interconnecting branches may become too high for a fully on-chip implementation. The passive segments or interconnects may have to reside on a PCB rather than in an IC.
As alluded, the distributed amplifiers may require the passive segments of the output transmission line to be on PCB, or namely, off-chip. This is more likely when the goal is to create a large signal or power-amplifier as opposed to a small-signal distributed amplifier. As described above, in the case of the power distributed amplifier, the amplified RF signal at each output power-transistor-section may be brought off-chip before connecting to a corresponding feed-point, tap-point, or summing-point on the output transmission line. Thus, the output power contribution at these locations bridges the on-chip/off-chip boundary at each power transistor. This can solve a dissipation problem in the output transmission line, but creates a new problem as well. Namely this approach distorts the locus or set of best load impedances presented to each constituent power transistor. This distortion is due to undesired parasitic-loading effects by presence of chip boundary or packaged transistor boundary, which is coupled to the output of the power transistor in this case. This distortion, made up of impedance-displacement and dispersion over frequency, mismatches the output impedance loading of the power transistors, which in-turn, likely degrades the performance of the distributed amplifier.
Thus, there is a need to compensate the load impedance at the feed-points, tap-points or summing points of a distributed amplifier that uses an off-chip output transmission line.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.